While autoclaving is an old procedure for which many types of structures have been designed, in general, the equipment is large, relatively expensive and relatively slow in operation. A particular difficulty is encountered in the sterilization of culture media such as agar where it has usually been the practice to prepare an agar solution in a glass flask and then to place the flask in an autoclave. Since it is difficult, if not impossible, to provide for stirring under such circumstances, heat transfer through the medium has been poor. This results in part from the fact that glass is a relatively poor heat transfer medium and in part from the fact that the agar solution is viscous so that heat transfer by convection is likewise poor.
Depending on the size of a flask, the contents of which it is desired to sterilize, the procedure may require as long as 2 to 3 hours. Since agar, as well as other culture media, is heat-labile, some degradation of the medium occurs. Consequently, it would be highly desirable that means be provided for raising the temperature of the medium to the desired level very quickly and insuring that the entire contents of the vessel stay at the requisite uniform temperature for an adequate length of time, while avoiding the degradation which is consequent on prolonged heating at sterilization temperature.
Attempts to meet this objective by the use of metal vessels fitted with electrical heaters have been made. However, local hot-spots can develop so that the temperature of the vessel wall may far exceed the 250.degree.F (121.degree.C) sterilization temperature. Moreover, if part of the electrically heated vessel loses contact with the solution in the vessel as the level drops due to removal of solution, or if the vessel is only partly filled, then local overheating and charring of heat-labile medium will occur.
Another problem encountered with conventional equipment arises from the fact that considerable heat is stored in the unjacketed dome or head of the vessel. After sterilization is complete and during the cooling-down period, some of this heat is radiated to the surface of the liquid in the vessel. Unless an agitator is present to provide adequate mixing, the medium at the surface of the solution can be seriously degraded.
Most sterilization systems in which the quantity of solution treated is larger than will be used in a single culture vessel require a dispensing port. Unless the port is properly jacketed, the material in the port will not be sterilized. Consequently, means for taking any solution in the dispensing portion of the system up to sterilization temperature and maintaining it there for the appropriate length of time must be provided.
As is evident from the aforenoted presentation, what is needed is a sterilizer which can be rapidly brought to sterilization temperature, which is free of hot-spots and which can rapidly be brought down to dispensing temperature and which insures that all of the culture medium in same is aseptic and free of degradation as the result of overheating.